During conventional applications of photoresist coatings to semiconductor wafers, a "coater" system is used. One part of the coater system is a flat, circular, disk-shaped, rotating vacuum chuck having a diameter slightly less than that of a semiconductor wafer. The vacuum chuck is used to hold and rotate a semiconductor wafer during the photoresist application process. The vacuum chuck is oriented such that a semiconductor wafer placed thereon resides in a level horizontal plane. In operation, the bottom or inactive surface of a semiconductor wafer is placed onto the vacuum chuck. The vacuum chuck applies a suction or negative pressure to the bottom surface of the semiconductor wafer to hold the semiconductor wafer on the vacuum chuck.
Commonly, a desired amount of liquid photoresist is applied to the top upwardly-facing surface of the semiconductor wafer while the semiconductor wafer is being rotated on the vacuum chuck. Thus, as the semiconductor wafer is rotating, the photoresist material spreads radially outward from the center of the semiconductor wafer towards the edge of the semiconductor wafer such that the entire top or active surface of the wafer is coated with a layer of photoresist. Excess photoresist material is sloughed off of the wafer during the rotation process.
However, excess amounts of photoresist tend to accumulate and form a mound or bead of photoresist on the outer edge of the semiconductor wafer. In order to eliminate the "edge bead" of photoresist, an edge bead removal unit is employed. Two types of edge bead removal units are well known in the art, chemical and optical. In a chemical edge bead removal unit, a nozzle dispenses a solvent referred to as edge bead removal fluid onto the photoresist at the edge of the semiconductor wafer. The solvent dissolves the photoresist and allows for easy removal of the photoresist from the edge of the semiconductor wafer. In an optical edge bead removal unit, the photoresist at or near the edge of the semiconductor wafer is exposed to light. During subsequent development processes, the exposed photoresist is removed. Photoresist which remains on the semiconductor wafer or overlying substrates forms a mask for subsequent processing operations. Unfortunately, prior art processes commonly inadvertently remove too much photoresist from the edge of the semiconductor wafer thereby exposing substrate layers to undesirable etching operations.
In the prior art, several edge bead removal units are utilized during fabrication of integrated circuit devices on the semiconductor wafer. The use of different edge bead removal units commonly results in a random or haphazard stacking of substrates layers at the edge of the semiconductor wafer. The randomly or haphazardly stacked substrate layers can lift and detrimentally redeposit onto the semiconductor wafer. The redeposited substrate material contaminates the semiconductor wafer and causes defects in the integrated circuit devices formed on the semiconductor wafer. Also, prior art processes which generate random or haphazard stacking of substrates layers at the edge of the semiconductor wafer often leave certain substrate layers detrimentally exposed to the ambient.
Furthermore, random or haphazard stacking of the substrate layers can also result in having a substrate layer inadvertently placed in contact with an underlying layer to which the overlying layer will not stick. In such an instance, the overlying layer will often peel form the underlying layer and detrimentally redeposit onto the semiconductor wafer. For example, a metal layer placed directly on top of a polysilicon layer will often peel from the polysilicon layer, contaminate the semiconductor wafer, and cause defects in the integrated circuit devices formed on the semiconductor wafer. Thus, it is desired that substrate layers are orderly stacked with overlying layers placed in contact only with underlying layers to which the overlying layers will stick.
Additionally, photoresist must be completely removed from the edge of the semiconductor wafer in order to prevent particulate contamination and subsequent defects. More specifically, a semiconductor wafer is commonly held in place or "clamped" during processing operations such as, for example, etching, sputtering and certain deposition operations. The clamp used to hold the semiconductor wafer in place is applied to the edge of the semiconductor wafer. Likewise, many semiconductor wafer handling devices also contact or clamp the edge of the semiconductor wafer. Because photoresist is a soft material, it tends to stick to clamps or wafer handling equipment, with which it comes into contact. After contacting the clamps or wafer handling equipment, the photoresist can flake off of the clamps or wafer handling equipment and redeposit onto the semiconductor wafer. The redeposited photoresist contaminates the semiconductor wafer and can cause defects in the integrated circuit devices formed on the semiconductor wafer.
In an attempt to avoid contacting the photoresist, prior art methods disclose expanding the region over which edge bead removal is applied. Such an approach is described by Hause et al., "Yield Improvement by Wafer Edge Engineering" SPIE vol. 2635, at 22-29 (August 1995). Such an approach expands the area from which photoresist is removed, thereby reducing the chance that any semiconductor wafer clamping or handling equipment contacts photoresist. However, such an approach reduces semiconductor wafer real estate available for the production of integrated circuit devices.
Thus, the need has arisen for a method or process which allows for an orderly arrangement of substrate layers at or near the edge of a semiconductor wafer thereby preventing undercutting of deposited substrate layers, preventing exposure of the deposited substrate layers to unwanted etching operations, and protectively sealing of the edges of certain deposited substrate layers from the ambient. A further need exists for a method or process which prevents unwanted photoresist contamination of semiconductor wafer clamping and handling equipment without significantly reducing semiconductor wafer real estate.